Countdown timer in minutes:seconds:milliseconds

 using UnityEngine;
 using System.Collections;
 using System.Collections.Generic;
 
 public class CountdownTimer : MonoBehaviour
 {
     public float m_startingMinutes = 3.0f; //in seconds
     public bool m_startTimer = false;
     public GUIText m_timerLabel; //drag GUIText in the scene here
 
     private float m_miliseconds;
     private float m_seconds;
     private float m_mins;
     private float m_totalmiliseconds;
 
     //-----------------------------------------------------------------------------------
     //Note: If you had a way to take in a username or initials you could 
     //do something along the lines of the below code instead of just using 
     //a list
     //
     //private Dictionary<string, int> m_scoreDictionary = new Dictionary<string, int>();
     //
     //With a dictionary you could link a name to a score value, just a thought :P
     //For now this script just uses the list set up below.
     //-----------------------------------------------------------------------------------
     private List<int> m_scores = new List<int>();
 
     // Use this for initialization
     void Start()
     {
         this.Init(m_startingMinutes);
     }
 
     // Update is called once per frame
     void Update()
     {
         if (m_startTimer && m_totalmiliseconds >= 0)
         {
             if (m_miliseconds <= 0)
             {
                 if (m_seconds <= 0)
                 {
                     m_mins--;
                     m_seconds = 59;
                 }
                 else
                 {
                     m_seconds--;
                 }
 
                 m_miliseconds = 99;
             }
 
             m_miliseconds -= Time.deltaTime * 100;
             m_totalmiliseconds -= Time.deltaTime * 100;
         }
         else if (m_totalmiliseconds <= 0)
         {
             m_miliseconds = 0.0f;
             m_seconds = 0.0f;
             m_mins = 0.0f;
         }
 
         if ((int)m_miliseconds > 9)
         {
             m_timerLabel.text = string.Format("{0}:{1}:{2}", m_mins, m_seconds, (int)m_miliseconds);
         }
         else
         {
             m_timerLabel.text = string.Format("{0}:{1}:0{2}", m_mins, m_seconds, (int)m_miliseconds);
         }
     }
 
     /// <summary>
     /// Public function to initialize the timer
     /// </summary>
     /// <param name="p_startingTime"></param>
     public void Init(float p_startingTime)
     {
         //On the note of PlayerPrefs, you may want to read them in here on the initialize      
         m_totalmiliseconds = p_startingTime * (60/*seconds*/) * (100/*miliseconds*/);
         m_mins = p_startingTime;
         m_startTimer = true;
     }
 
     /// <summary>
     /// Public function to pause the timer
     /// </summary>
     /// <param name="p_pause"></param>
     public void PauseTimer(bool p_pause)
     {
         m_startTimer = p_pause;
     }
 
     /// <summary>
     /// Store the score in a list
     /// </summary>
     public void StoreScore()
     {
         //Store the total milisceconds left in as the score so it we want to
         //use it later we have a number we can format back into MM:SS:mm
         this.m_scores.Add((int)m_totalmiliseconds);
 
         //Something to consider here is that these score will not be persistent 
         //between games, you may want to consider using the PlayerPrefs to store
         //a string that can be parsed out and fed back into the m_scores list.
         //link: http://docs.unity3d.com/Documentation/ScriptReference/PlayerPrefs.html
         
     }
 }

Mines quite similar to the one above. Just posting mine because it gives you the ability to start and stop it and set the time publicly so you could hook it into other scripts quite easily. Just thought the additions might be welcome :)

Cheers and good luck,

Hans